1. Field of the Invention
This invention relates to a plastic lens for spectacles which has high refractive index, transparency and heat resistance.
2. Description of Prior Art
A plastic lens is advantageous over a glass lens because it is light, easy to dye, and safe when it is used as a spectacle lens because of its high impact resistance.
Various resins such as an acrylic resin, a polycarbonate resin, an allyl diglycolcarbonate resin, a polystyrene resin and the like have heretofore been used for a plastic lens. Among these, diethylene glycol bisallylcarbonate, which is a thermosetting resin, is widely used for an ophthalmic lens because it has several advantages, such as high impact resistance, excellent cutting property and processability.
However, the lens of this resin is thicker at the peripheral edge portion than that of a glass lens because the refractive index of diethylene glycol bisallylcarbonate is about 1.50. This defect is more prominent as the degree of the lens is increased.
Various resins for plastic lenses free from the abovementioned defect have recently been proposed. For example, British Pat. No. 2,034,721 teaches that a copolymer of a dimethacrylate having a bisphenol group with chlorostyrene has a refractive index of 1.591. Japanese Patent Publication No. 14449/83 teaches that a copolymer of a dimethacrylate having a tetrabromobisphenol group which chlorostyrene has a refractive index of 1.599 to 1.608. Laid-Open Japanese Patent Publication No. 12501/85 teaches that a copolymer of a tetrachloro diallylterephthalate with chlorostyrene has a refractive index of 1.600 to 1.609 and Laid-Open Japanese Patent Publication No. 14202/85 teaches a copolymer of a tetrabromo diallylphthalate with phenoxyethylacrylate has a refractive index of 1.595.
Though these resins have higher refractive index that that of crown glass (nd=1.52), they are inferior in toughness and heat resistance. Further, since the monomer mixture containing the above mentioned nucleus-halogensubstituted polyfunctional vinyl monomer used as a high refractive index component is viscous because of its high molecular weight the casting process is not easy.
For imparting a good toughness, mixing or copolymerizing these resin with polyurethane has been known. For example, U.S. Pat. No. 4,487,904 teaches that a copolymer of a nucleus-halogen substituted vinyl monomer (containing hydroxyl groups) and polyfunctional isocyanate with another vinyl monomer shows a refractive index of 1.61 to 1.63 together with good toughness.
However, the refractive index of these copolymers is still lower than that of high refractive index glass lenses (nd=1.6 to 1.8). Moreover, in ordinary cases, whenever the refractive index is increased, the Abbe number is decreased (the chromatic aberration is increased), which is disadvantageous.